Is array concatenation a monoid?

I'm not so sure about the monoid section, where it's stated that:

> Array concatenation also forms a monoid

With the example:

```js
[1, 2].concat([3, 4])
```

I don't feel like array concatenation satisfies "associativity" at all, since `a.concat(b)` clearly differs from `b.concat(a)`. Can someone explain this for me?